[PATCH 3/3] mm: remove tmem specifics from frontswap

[Juergen Gross]

The frontswap module contains several parts which are specific to
tmem. With that no longer present those parts can be removed.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
 Documentation/vm/frontswap.rst |  17 +----
 include/linux/frontswap.h      |   5 --
 mm/Kconfig                     |  16 ++---
 mm/frontswap.c                 | 156 +----------------------------------------
 4 files changed, 7 insertions(+), 187 deletions(-)

diff --git a/Documentation/vm/frontswap.rst b/Documentation/vm/frontswap.rst
index 511c921bc8d2..2c674c0c6a77 100644
--- a/Documentation/vm/frontswap.rst
+++ b/Documentation/vm/frontswap.rst
@@ -19,7 +19,7 @@ for a detailed overview of frontswap and related kernel parts)
 Frontswap is so named because it can be thought of as the opposite of
 a "backing" store for a swap device.  The storage is assumed to be
 a synchronous concurrency-safe page-oriented "pseudo-RAM device" conforming
-to the requirements of transcendent memory (such as Xen's "tmem", or
+to the requirements of transcendent memory (such as
 in-kernel compressed memory, aka "zcache", or future RAM-like devices);
 this pseudo-RAM device is not directly accessible or addressable by the
 kernel and is of unknown and possibly time-varying size.  The driver
@@ -113,21 +113,6 @@ many servers in a cluster can swap, dynamically as needed, to a single
 server configured with a large amount of RAM... without pre-configuring
 how much of the RAM is available for each of the clients!
 
-In the virtual case, the whole point of virtualization is to statistically
-multiplex physical resources across the varying demands of multiple
-virtual machines.  This is really hard to do with RAM and efforts to do
-it well with no kernel changes have essentially failed (except in some
-well-publicized special-case workloads).
-Specifically, the Xen Transcendent Memory backend allows otherwise
-"fallow" hypervisor-owned RAM to not only be "time-shared" between multiple
-virtual machines, but the pages can be compressed and deduplicated to
-optimize RAM utilization.  And when guest OS's are induced to surrender
-underutilized RAM (e.g. with "selfballooning"), sudden unexpected
-memory pressure may result in swapping; frontswap allows those pages
-to be swapped to and from hypervisor RAM (if overall host system memory
-conditions allow), thus mitigating the potentially awful performance impact
-of unplanned swapping.
-
 A KVM implementation is underway and has been RFC'ed to lkml.  And,
 using frontswap, investigation is also underway on the use of NVM as
 a memory extension technology.
diff --git a/include/linux/frontswap.h b/include/linux/frontswap.h
index 6d775984905b..052480aa3756 100644
--- a/include/linux/frontswap.h
+++ b/include/linux/frontswap.h
@@ -24,11 +24,6 @@ struct frontswap_ops {
 };
 
 extern void frontswap_register_ops(struct frontswap_ops *ops);
-extern void frontswap_shrink(unsigned long);
-extern unsigned long frontswap_curr_pages(void);
-extern void frontswap_writethrough(bool);
-#define FRONTSWAP_HAS_EXCLUSIVE_GETS
-extern void frontswap_tmem_exclusive_gets(bool);
 
 extern bool __frontswap_test(struct swap_info_struct *, pgoff_t);
 extern void __frontswap_init(unsigned type, unsigned long *map);
diff --git a/mm/Kconfig b/mm/Kconfig
index 5166fe4af00b..971b615ad3a6 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -436,20 +436,14 @@ config NEED_PER_CPU_KM
 	default y
 
 config FRONTSWAP
-	bool "Enable frontswap to cache swap pages if tmem is present"
+	bool "Enable frontswap to cache swap pages if zswap is present"
 	depends on SWAP
 	help
 	  Frontswap is so named because it can be thought of as the opposite
-	  of a "backing" store for a swap device.  The data is stored into
-	  "transcendent memory", memory that is not directly accessible or
-	  addressable by the kernel and is of unknown and possibly
-	  time-varying size.  When space in transcendent memory is available,
-	  a significant swap I/O reduction may be achieved.  When none is
-	  available, all frontswap calls are reduced to a single pointer-
-	  compare-against-NULL resulting in a negligible performance hit
-	  and swap data is stored as normal on the matching swap device.
-
-	  If unsure, say Y to enable frontswap.
+	  of a "backing" store for a swap device.  The only user right now is
+	  zswap.
+
+	  If unsure, say "n".
 
 config CMA
 	bool "Contiguous Memory Allocator"
diff --git a/mm/frontswap.c b/mm/frontswap.c
index 157e5bf63504..e3370e46a0a5 100644
--- a/mm/frontswap.c
+++ b/mm/frontswap.c
@@ -33,23 +33,6 @@ static struct frontswap_ops *frontswap_ops __read_mostly;
 #define for_each_frontswap_ops(ops)		\
 	for ((ops) = frontswap_ops; (ops); (ops) = (ops)->next)
 
-/*
- * If enabled, frontswap_store will return failure even on success.  As
- * a result, the swap subsystem will always write the page to swap, in
- * effect converting frontswap into a writethrough cache.  In this mode,
- * there is no direct reduction in swap writes, but a frontswap backend
- * can unilaterally "reclaim" any pages in use with no data loss, thus
- * providing increases control over maximum memory usage due to frontswap.
- */
-static bool frontswap_writethrough_enabled __read_mostly;
-
-/*
- * If enabled, the underlying tmem implementation is capable of doing
- * exclusive gets, so frontswap_load, on a successful tmem_get must
- * mark the page as no longer in frontswap AND mark it dirty.
- */
-static bool frontswap_tmem_exclusive_gets_enabled __read_mostly;
-
 #ifdef CONFIG_DEBUG_FS
 /*
  * Counters available via /sys/kernel/debug/frontswap (if debugfs is
@@ -167,24 +150,6 @@ void frontswap_register_ops(struct frontswap_ops *ops)
 }
 EXPORT_SYMBOL(frontswap_register_ops);
 
-/*
- * Enable/disable frontswap writethrough (see above).
- */
-void frontswap_writethrough(bool enable)
-{
-	frontswap_writethrough_enabled = enable;
-}
-EXPORT_SYMBOL(frontswap_writethrough);
-
-/*
- * Enable/disable frontswap exclusive gets (see above).
- */
-void frontswap_tmem_exclusive_gets(bool enable)
-{
-	frontswap_tmem_exclusive_gets_enabled = enable;
-}
-EXPORT_SYMBOL(frontswap_tmem_exclusive_gets);
-
 /*
  * Called when a swap device is swapon'd.
  */
@@ -280,9 +245,6 @@ int __frontswap_store(struct page *page)
 	} else {
 		inc_frontswap_failed_stores();
 	}
-	if (frontswap_writethrough_enabled)
-		/* report failure so swap also writes to swap device */
-		ret = -1;
 	return ret;
 }
 EXPORT_SYMBOL(__frontswap_store);
@@ -314,13 +276,8 @@ int __frontswap_load(struct page *page)
 		if (!ret) /* successful load */
 			break;
 	}
-	if (ret == 0) {
+	if (ret == 0)
 		inc_frontswap_loads();
-		if (frontswap_tmem_exclusive_gets_enabled) {
-			SetPageDirty(page);
-			__frontswap_clear(sis, offset);
-		}
-	}
 	return ret;
 }
 EXPORT_SYMBOL(__frontswap_load);
@@ -369,117 +326,6 @@ void __frontswap_invalidate_area(unsigned type)
 }
 EXPORT_SYMBOL(__frontswap_invalidate_area);
 
-static unsigned long __frontswap_curr_pages(void)
-{
-	unsigned long totalpages = 0;
-	struct swap_info_struct *si = NULL;
-
-	assert_spin_locked(&swap_lock);
-	plist_for_each_entry(si, &swap_active_head, list)
-		totalpages += atomic_read(&si->frontswap_pages);
-	return totalpages;
-}
-
-static int __frontswap_unuse_pages(unsigned long total, unsigned long *unused,
-					int *swapid)
-{
-	int ret = -EINVAL;
-	struct swap_info_struct *si = NULL;
-	int si_frontswap_pages;
-	unsigned long total_pages_to_unuse = total;
-	unsigned long pages = 0, pages_to_unuse = 0;
-
-	assert_spin_locked(&swap_lock);
-	plist_for_each_entry(si, &swap_active_head, list) {
-		si_frontswap_pages = atomic_read(&si->frontswap_pages);
-		if (total_pages_to_unuse < si_frontswap_pages) {
-			pages = pages_to_unuse = total_pages_to_unuse;
-		} else {
-			pages = si_frontswap_pages;
-			pages_to_unuse = 0; /* unuse all */
-		}
-		/* ensure there is enough RAM to fetch pages from frontswap */
-		if (security_vm_enough_memory_mm(current->mm, pages)) {
-			ret = -ENOMEM;
-			continue;
-		}
-		vm_unacct_memory(pages);
-		*unused = pages_to_unuse;
-		*swapid = si->type;
-		ret = 0;
-		break;
-	}
-
-	return ret;
-}
-
-/*
- * Used to check if it's necessory and feasible to unuse pages.
- * Return 1 when nothing to do, 0 when need to shink pages,
- * error code when there is an error.
- */
-static int __frontswap_shrink(unsigned long target_pages,
-				unsigned long *pages_to_unuse,
-				int *type)
-{
-	unsigned long total_pages = 0, total_pages_to_unuse;
-
-	assert_spin_locked(&swap_lock);
-
-	total_pages = __frontswap_curr_pages();
-	if (total_pages <= target_pages) {
-		/* Nothing to do */
-		*pages_to_unuse = 0;
-		return 1;
-	}
-	total_pages_to_unuse = total_pages - target_pages;
-	return __frontswap_unuse_pages(total_pages_to_unuse, pages_to_unuse, type);
-}
-
-/*
- * Frontswap, like a true swap device, may unnecessarily retain pages
- * under certain circumstances; "shrink" frontswap is essentially a
- * "partial swapoff" and works by calling try_to_unuse to attempt to
- * unuse enough frontswap pages to attempt to -- subject to memory
- * constraints -- reduce the number of pages in frontswap to the
- * number given in the parameter target_pages.
- */
-void frontswap_shrink(unsigned long target_pages)
-{
-	unsigned long pages_to_unuse = 0;
-	int uninitialized_var(type), ret;
-
-	/*
-	 * we don't want to hold swap_lock while doing a very
-	 * lengthy try_to_unuse, but swap_list may change
-	 * so restart scan from swap_active_head each time
-	 */
-	spin_lock(&swap_lock);
-	ret = __frontswap_shrink(target_pages, &pages_to_unuse, &type);
-	spin_unlock(&swap_lock);
-	if (ret == 0)
-		try_to_unuse(type, true, pages_to_unuse);
-	return;
-}
-EXPORT_SYMBOL(frontswap_shrink);
-
-/*
- * Count and return the number of frontswap pages across all
- * swap devices.  This is exported so that backend drivers can
- * determine current usage without reading debugfs.
- */
-unsigned long frontswap_curr_pages(void)
-{
-	unsigned long totalpages = 0;
-
-	spin_lock(&swap_lock);
-	totalpages = __frontswap_curr_pages();
-	spin_unlock(&swap_lock);
-
-	return totalpages;
-}
-EXPORT_SYMBOL(frontswap_curr_pages);
-
 static int __init init_frontswap(void)
 {
 #ifdef CONFIG_DEBUG_FS
-- 
2.16.4